Focused ion beam assisted analysis of the oxidation of a NiAl coating on pure Ni

Discs of (pure nickel 15 mm diameter and 4 mm thickness) have been polished to a 1 μm finish, aluminised using a standard pack CVD process to give a β-NiAl coating and then the surface of this coating polished to a 1 μm finish again. Samples have then been isothermally oxidised in a muffle furnace for 1, 2, 4, 8, 16, 32, 64, 128, 256, 512 and 1024 hours at 950°C, and subjected to surface and cross-sectional microscopy and analysis. Scales were also cross-sectioned and imaged using Focused Ion Beam Milling. The oxidation rate was determined to be 2.6 × 10-¹³ g² cm ^-4 s^-1 and for times less than 512 hours the predominant alumina phase was θ alumina. Al depletion effects were observed due to both oxidation and interdiffusion between the Ni substrate and the NiAl coating. Using these effects, it has been shown that tensile strains of up to 9% can arise and these strains readily explain the formation of intrefacial elliptical pores observed by FIB cross sectioning. FIB sectioning also indicated that different coating grains exhibit different oxide-coating interfaces which arise due to different grain orientations. The faceting of the coating-oxide interface and the development of cuboidal faceted coating structures beneath spalled oxide is thought to be due to substructure development (sub-grain development or recrystallisation) arising from the plastic strains generated by Al depletion effects.